It is well known to use a seam sealing process to attach metal covers to the seal surface of ceramic chip carriers to provide a hermetically sealed package for electronic components such as a surface acoustic wave filter. The carriers are normally rectangular or square and have a metal surface to accept the metal cover. The length and width of the cover match the length and width of the sealing surface on the ceramic carrier to within 0.001 inches. The sealing surface on the package is a metal frame that is brazed onto a metallized pattern on the uppermost ceramic layer of the carrier so that the metal seal ring becomes the uppermost surface of the ceramic carrier. The opening in the center of the seal ring typically matches the opening in the ceramic carrier that accepts the electrical components.
In the sealing process the cover is accurately located on the seal ring (typically by placing the cover with a mechanism that is guided by a vision or pattern recognition system). As the cover is placed, it is lightly tack-welded into place in order to maintain the relative position of the cover to the seal ring through seam weld sealing.
The seam sealing process incorporates the use of fixturing to hold a matrix of multiple packages and rotate the matrix of packages 90 degrees in order to obtain seam sealing along the length and width of a rectangular shaped package. Typically, the seam sealer or welder has 2 tapered metal rollers as electrodes that actually contact the cover during seal. The rollers are energized so that electrical current passes from one roller, through the cover and seal ring combination, and finally into the other roller. This electrical current passing through the metal components results in heat generation (current squared.times.resistance=watts-heat energy) within the various metal components.
As heat is generated at various locations during the sealing process the temperature of each component will change relative to the amount of heat, the electrical resistivity, and the heat capacity of the component parts. Since heat flow is driven by temperature differences then the heat flow and temperature changes will be directly effected by the starting temperatures of all components that are subjected to the heat. Thus, it is unlikely that all components will be at the same temperature when serially sealing multiple parts.
By way of example, temperature control of electrodes is known as evidenced in U.S. Pat. Nos. 5,089,682 to Davies and 2,407,676 to Munson, by way of example. However, it needs to be understood that sealing parameters such as roller pressure, seal current, roller temperature, fixture temperature, package temperature, roller velocity, current pulse duration and frequency all must be controlled and be repeated from package to package in order to have a truly controlled process that will yield high percentages of hermetically sealed parts within packages without defects in their seal.
In the typical seam sealing scenario, the temperature of all components generally starts low (maybe near room temperature) and climbs slowly (uncontrollably) with each additional package that is sealed. This requires that the seal parameters be modified repeatedly as the sealing process is repeated through some large number of packages that are typical in a hi-volume production situation. By way of example, as the first package is sealed all components (package fixture, carrier, cover, rollers, roller mounts, etc.) are near ambient temperature. With each additional package that is welded the temperature of each component changes. Eventually, the hermetic seal of one of the packages is corrupted by the now out of specification seal parameters so weld parameters must be adjusted to yield a desired seal with all of the new temperatures in the system.
Temperature is important since this is a sealing process that incorporates the melting and mutual reflow of some portion of the cover and seal ring to accomplish the hermetic seal. It is ultimately important to achieve a liquid state in the parts to be joined without the temperature being excessive.
As the components of the sealing system change temperature this means that the optimum seal conditions are only present for a few (maybe only one) packages and that these conditions change as the various temperatures change until the seal conditions are grossly out of spec and the seal quality is poor. After a parameter change to compensate for these new temperature the first few packages may have optimum settings but quickly the sealing or welding conditions will degrade as temperature changes. These temperature changes can be up or down. The temperature will typically go up as parts are sealed one after the other and heat is continually generated within the systems components but the temperature can also go down. By way of example, a welder operator goes to lunch or takes a break and thus creates a situation where the temperature would rise or fall at random.
So, simply stated, desirable sealing conditions exist for only a very small percentage of packages within the matrix of packages being welded, a problem in the art. In addition, a new problem has been identified that compounds the package sealing difficulties. It is the stress that is induced in the cover and carrier combination during and after the welding or sealing process that is caused by a mismatch of the coefficients of thermal expansion of the various materials that make up the carrier and cover. When the carrier and cover are ready for sealing they are each at some temperature and typically they are usually at the same temperature. Although they could be at any temperature for various reasons. By way of example, assume that they are at nearly the same temperature, this being the most common situation.
As the sealing process begins, the temperature of the cover and carrier, the package, begin to rise during this sealing process they will both rise, most likely not the same amount, until they reach their respective maximums and then they cool down after the sealing by the seam welding process has been completed. In this cool down period the cover and carrier are intimately joined and will cool at very near if not exactly the same rate and finally settle somewhere close to an ambient temperature. Since they have differing coefficients of thermal expansion, they did not change dimension or size at an equal rate and thus the ultimate change in size of the cover is not the same as it is for the carrier. So each element is attempting to cause the other to conform to its size. This results in a mechanical stress in the package, the carrier and cover assembly. In other words if you consider that some amount of force is required to cause the package to fracture, this stress induced by the cool down after sealing, attaching the cover to the carrier through a seam welding, has now decreased the total stress from outside sources that would be required to fracture the package.
The total stress required to fracture the package is now typically equal to the total stress minus the seal induced stress. Tests carried out at Sawtek, Inc. have shown this to be the case. Package fracture problems have been observed on surface mount ceramic packages. The ceramic package was subjected to an outside mechanical stress and repeatedly resulted in package cracking. A device was built to accurately reproduce the outside stress and it was found that empty packages without a cover required more mechanical stress to create a fracture than did those packages that had sealed covers in place.
There is a need to reduce and even eliminate the induced stress resulting from the differing coefficients of thermal expansion (CTE).